IC cards with CPUs featuring security functions, personal identification functions and the like are broadly divided into “IC cards with contacts” which communicate data with a reader/writer via contacts, and “contactless IC cards” which perform data transmission by electromagnetic induction or the like. Among these IC cards, contactless IC cards which transmit data via radio have greater durability because they do not need a connecting terminal to connect to an external device. Further, such contactless IC card rectifies received waves using a rectifier to generate a DC power supply that is required to activate the integrated circuit, eliminating the need of batteries, whereby it is effective in miniaturization of the system and reduction of the costs.
The conventional contactless IC card includes an analog circuit, a CPU, or a memory on one integrated circuit (for example, refer to “A 13.56 MHz CMOS RF Identification Transponder Integrated Circuit WithA Dedicated CPU” (Shoichi Masuiet al., ISSCC Digest of Technical Papers, pp. 162–163, FIG. 9.1.1 (Feb. 16, 1999)). There are also contactless IC cards to which a power supply voltage is supplied with stability even when a relative position between a reader/writer and the IC card varies (for example, refer to Japanese Patent No. 3376085, FIG. 3).
The operation of such contactless IC card will be described with reference to FIG. 9. A contactless IC card 1 comprises a coil antenna L1 and a semiconductor integrated circuit 2. The semiconductor integrated circuit 2 comprises a rectifier 3, a shunt regulator 4, a demodulator 5, a modulator 6, a digital signal processing unit 7, a linear regulator 8, and a reference voltage circuit 9. As the rectifier 3, a full-wave rectification circuit that employs diodes D1 to D4 as shown in FIG. 10 is used.
A signal that is received by the coil antenna L1 is rectified by the rectifier 3 to generate a power supply voltage VDDA. The demodulator 5 demodulates RX (receiving) data which is superimposed upon the power supply voltage VDDA. The RX data is transferred to the digital signal processing unit 7, which is constituted by a CPU or a memory. The modulator 6 modulates an impedance between ends of the coil antenna L1 in accordance with TX (transmission) data that is generated by the digital signal processing unit 7. As the reference voltage circuit 9, a band-gap reference circuit as shown in FIG. 11 is used. This circuit generates a reference voltage Vref. In the case of band-gap reference circuit, this circuit generates, for example, the reference voltage Vref=1.2V.
As the linear regulator 8, a regulator circuit that employs an operational amplifier as shown in FIG. 8 is used. In the case of linear regulator as shown in FIG. 8, a power supply voltage VDDD having a value of Vref×(1+R1/R2) is generated as an output. For example, when it is assumed R1=R2, VDDD=2.4V. The power supply voltage VDDD is a power supply voltage for the digital signal processing unit 7.
The shunt regulator 4 is a circuit that prevents the power supply voltage VDDA from increasing above a breakdown voltage. It is assumed here that the communication standard is ISO14443 TYPE B. According to this standard, the carrier frequency is 13.56 MHz, the data rate is 106 kbps, the data transmission from the reader/writer to the contactless IC card is done by means of the 10% ASK modulation, and the data transmission from the contactless IC card to the reader/writer is done by means of the BPSK modulation.
The power that is supplied to the contactless IC card is decided based on the intensity of a magnetic field that is applied to the card coil. Usually, when the card becomes closer to the reader/writer (not shown), the intensity of the magnetic field is increased, whereby the power that is supplied to the semiconductor integrated circuit 2 is increased. The supplied power is converted into a DC voltage by the rectifier 3. Here, when the load to the semiconductor integrated circuit 2 is fixed, the power supply voltage is increased in proportion to the supplied power. The breakdown voltage of a transistor which is manufactured in the present semiconductor process is about 5V when the thickness of the gate oxide film is 10 nm. When the power supply voltage VDDA is increased above the breakdown voltage, the transistor would be broken.
The shunt regulator 4 that consumes an unnecessary power is employed to suppress an increase of the power supply voltage VDDA. For example, when the power supply voltage is increased above 4V, the shunt regulator 4 consumes excess energy and, as a result, the increase of the power supply voltage VDDA can be reduced. Further, the capability of the shunt regulator 4 is adjusted suitably to demodulate a modulated signal by the demodulator 5.
The conventional contactless IC card is constructed as described above and, since there is no need for a connecting terminal to connect to an external device, it has greater durability, and further, as the batteries are not required, this is effective in miniaturization of the system or reduction of the costs. However, this conventional IC card has the following problem. The linear regulator 8 cannot supply the power supply voltage VDDD earlier than start-up of the reference voltage circuit 9. This is because when the reference voltage Vref=0V, the voltage output from the above-mentioned linear regulator 8 becomes a power supply voltage VDDD=0. The start-up of the reference voltage circuit 9 takes time of above 100 μsec. For the above-mentioned reasons, when the energy that ought to be supplied to the power supply voltage VDDD is supplied to the power supply voltage VDDA, the potential of the power supply VDDA is increased, and when the power supply voltage voltage VDDA is increased above the breakdown voltage, the device would be broken. Such breakage of the device presents a more serious problem when the size of the digital signal processing unit 7 is larger, because the power supply voltage VDDA is increased more.
In order to suppress such increase of the power supply voltage VDDA, it is possible to increase the capacity of the shunt regulator 4, but when an ASK signal is to be demodulated, the demodulator 5 detects variations in the power supply voltage VDDA to demodulate RX data and, thus, when the capacity of the shunt regulator 4 is simply increased, the amount of variations in the signal is reduced, whereby the demodulation of the ASK signal by the demodulator 5 becomes difficult.